1. Field of the Invention
The present invention relates to a method for forming a thin film and to a semiconductor device and more particularly, to a method for forming a thin film such as TiN film which is formed as a barrier layer on the inside surface of a contact hole in a semiconductor device and to the semiconductor device in which the thin film was formed.
2. Description of the Relevant Art
A contact section in a semiconductor device such as a LSI device has a diffusion layer on the surface of a substrate as serving a base which is connected with an interconnection such as Al through a contact hole.
However, as LSI devices get further refined and more highly integrated, the diffusion layer formed on the surface of the substrate becomes shallower. Problems arise in the shallow diffusion layer such as an Al spike occurs and destroys a connection or Si deposits on the bottom of the contact hole, which results in increasing contact resistance.
In order to solve these problems, Al alloys (e.g. Al-1% Si) previously contaminated with about 0.5-2% of Si are used as the electrode connection material, but even these Al alloys recently are insufficient to prevent deposition of Si since the diameter of the contact hole is smaller than before.
Accordingly, forming a thin film called a barrier metal between the Al alloy and a Si substrate for prevention of diffusion has been considered. A TiN film is known as a barrier metal on account of its small electrical resistance and chemical stability (Yamanishi, Yoshihara, Kitahara, and Hosokawa; "Vacuum", Vol. 30 No. 5 P347, 1987).
A TiN film has so far been formed by the "Reactive Sputtering" method. The Reactive Sputtering method forms a TiN film on a substrate 31 by using Ti as a target 30 and providing Ar and N.sub.2 as a sputtering gas in such an device as shown in FIG. 21 (Kanamori; "Vacuum", Vol. 29 No. 9 P418, 1986). Reference numeral 32 designates a coil.
However, by this method, wherein step coverage is poor, a TiN film is formed in the contact section as shown in FIG. 22(a). As a semiconductor device gets more highly integrated to 64MDRAM, and next to 256MDRAM, the aspect ratio (hole depth/hole diameter) of contact hole 3 becomes gradually higher. As a result, by this method almost no TiN film is formed on the bottom of a contact hole 35, so that it becomes impossible to use this method. In other words, even if in a state shown in FIG. 22(a) where electrode material such as W(tungsten) or Al is filled in, a void 36 occurs inside the contact hole 35. As a result, the interconnection broken thereby resulting in losing its function as a barrier metal, so that it is impossible to maintain the reliability of the LSI devices produced.
Accordingly, the LPCVD method (Low Pressure Chemical Vapor Deposition method), one of the thermal CVD methods, began to be as a method for forming a thin film having good step coverage. This method forms a TiN film on a substrate by thermal reaction with using TiCl.sub.4, and NH.sub.3 or N.sub.2 gas as materials in such an device as shown in FIG. 23 (N. Yokoyama et al., Vol. 136 No. 3, J. Electrochem. Soc., P882)(N. Yokoyama et al., Vol. 138 No. 1, J. Electrochem. Soc., P190).
However, since the formation of a TiN film by reaction of TiCl.sub.4 +N.sub.2 +H.sub.2 requires a high temperature of about 900.about.1000.degree. C. which has an adverse influence on device characteristics, the method had a problem that it cannot be applied to production of a LSI device (Arthur Sherman, Extended Abstract of the 1991 International Conference on Solid State Devices and Materials, 1991 P177.about.179).
On the other hand, since the reaction of TiCl.sub.4 +NH.sub.3 takes place at a low temperature, a method wherein NH.sub.3 gas is used was promising for forming a barrier metal for a LSI device. However, there was a problem that TiCl.sub.4 and NH.sub.3 form a complex (TiCl.sub.4.nNH.sub.3) in the vapor-phase, which yield a yellow powder resulting in the generation of particles (M. J. Buiting, et al., J. Electrochem. Soc., Vol. 138, No. 2, February 1991 P500). Besides, by the LPCVD method, a TiN film is comformally formed in a contact section as shown in FIG. 24(a). In the case of the formation of the barrier metal by the LPCVD method, a TiN film 33 is sufficiently formed on the bottom of a contact hole 35, which is preferable as a barrier metal. However a TiN film of the same thickness as that on the bottom of the contact hole 35 is simultaneously formed on the side wall of the contact hole. The formation of a thick film of TiN on the side wall in this fashion results in a small diameter of the hole remaining in the contact hole 35. It therefore is substantially the same situation in which a contact hole of a large aspect ratio is formed. Consequently, in the next step, when W(tungsten) or Al and the like are filled in, a void 36 occurs in the contact hole 35 as shown in FIG. 24(b). This another problem so as to maintain reliability of a LSI device is created.
In the LPCVD method, such a device as shown in FIG. 23 is used as mentioned above. Reference numeral 40 designates a chamber wherein a gas nozzle 41 and a sample stage 42 are arranged, and on the sample stage 42, a substrate 43 is placed. The bottom of the chamber 40 is connected with a booster pump 44 and a rotary pump 45, and the gas nozzle 41 is connected with a pipe 47 for NH.sub.3 gas supply.